What you are referring to is quantity discounts.
10% off if you purchase 2
15% off if you purchase 3
20% off if you purchase 4
There are many apps on the Shopify ecosystem that allow you to enable this functionality. Without knowing more about your store, products or audience it is hard to make a recommendation. For example (percentage based discounts or $ based discounts on quantity size, BOGO, etc.) Coupon theory is awesome, and an incredibly powerful motivator to influence your audience to making more than one purchase.
Reach out to me any time and we can do through some of the details to determine what will work best for you.